In a conventional wireless system represented on FIG. 1, a primary station 100 exchanges data with a plurality of secondary stations 110. To transmit its data, the primary station 100 transmits sends its data, on a downlink data channel 101. This downlink data channel may be adjusted over the time in response to several criterions, e.g. Quality of Service, interference, downlink channel quality. To inform, the secondary stations 110 of these changes, the primary station 100 transmits control, data (or signalling) to the secondary stations 110 on a downlink control channel 102. Similarly, the secondary stations 110 transmit their data on an uplink control channel 111. Moreover, an uplink control channel 112 is used by the secondary station to request resources for transmissions and/or for providing the primary station, with feedback on the downlink transmissions or on the channel quality state.
In many wireless systems, like mobile communication systems as UMTS LTE (Long Term Evolution), or LTE Advanced, signalling on control channels 102 or 112 is provided so that it indicates the particular time-frequency transmission resources to which the data is mapped on the data channels 101 or 111, and the transmission scheme used for that data (i.e. the format/mode in which the data itself is transmitted). In addition the secondary station may provide channel state feedback intended to assist the primary station in scheduling transmission to suitable secondary stations using appropriate transmission, resources and transmission scheme. Therefore, in general a transmission mode may be defined by one or both of a transmission scheme used by the primary station or the type of feedback provided by the secondary station.
For UMTS LTE Rel-8 downlink communication, the relevant downlink control channel 101 that includes resource allocation and transmission format information is known as Physical Downlink Control Channel (PDCCH). The message a PDCCH carries is known as Downlink Control Channel Information (DCI). The resource allocations for different transmission modes are typically indicated using different DCI formats. The secondary station (here a User Equipment or UE) is configured to receive a limited number of different DCI formats torn the possible set. Therefore configuring the types of DCI format expected by the UE in the PDCCH directly controls the transmission modes which may be expected by the secondary station, and a particular transmission mode is signalled by the particular DCI format used for resource allocation. The DCI Format contents cap also be associated with a transmission mode (e.g. in LTE a 1 bit field indicated uplink or downlink resources).
Even with several possible transmission modes, it is desirable not to specify too many DCI formats in order to reduce implementation and testing complexity. On the other hand, in the further development of LTE it is likely that additional DCI formats will need to be defined. Of particular interest, now transmission modes are discussed like the Cooperative Multi-Point transmission (CoMP transmission) or add the Multi User-MIMO (MU-MIMO) mode currently being redefined to allow additional flexibility. These new transmission modes will require to be signalled. However, it is preferable not to increase the number of DCI formats. In addition, it is desirable to avoid signalling overhead.
In LTE the transmission mode (i.e. DCI formats expected by the UE and/or feedback mode) are configured by higher layer signalling. However, if frequent changes of transmission mode are required, the use of higher layer signalling may introduce an unacceptable delay in data transmission and a significant overhead due to the signalling messages.